The invention relates to a device for generating images of an examination subject with nuclear magnetic resonance in which means exist for applying magnetic fields to the examination subject and for detecting the excursion of the nuclear spins of the examination subject from its state of equilibrium due to a high frequency excitation pulse, and in which the means for applying magnetic fields exhibit gradient coils which are supplied by a control device and establish the position of the examined layer in the examination subject with reference to a three-dimensional coordinate system with the x-, y-, and z-axis.
Given a device of this type, the fact is exploited that the atomic nuclei of specific elements, particularly of hydrogen, are aligned in a static magnetic field and can be subject to an excursion from their preferred direction by means of a high frequency excitation pulse (returning into said preferred direction after the end of the excitation pulse). During the return, they execute a precessional motion around the preferred position. The information thereby generated in a measuring coil has a frequency which depends on the magnetic flux density.
It is also possible to achieve a selected excitation of a specific layer of the examination subject in that one exploits the fact that a specific frequency is required for the excitation of atomic nuclei, said frequency depending on the magnetic flux density. When a field gradient is superimposed on the magnetic field for the alignment of the atomic nuclei, then the atomic nuclei in only a specific layer of the examination subject in which the magnetic flux density corresponds to the frequency of the excitation pulse are excited.
It is known that one can generate gradient vectors by means of a suitable disposition of gradient coils, said gradient vectors, as a result of linear combination, producing a desired (randomly selectable) oriented sum vector and, therewith, selecting any desired layer in the examination subject. However, a relatively large outlay in terms of electronics in the control device for the gradient coils is required for the selection of a layer lying at a desired location in the examination subject.